My car is now five years old and i have never had to use the spare wheel but i was thinking how long can you keep a spare Tyre before the rubber deteriorates.
Anyone know.

Well they reckon to get them checked after about 5 years for cracking but its sunlight that ages tyres so a spare should last a long longer than one in use. I'd say 8-10 years if it never comes out of the boot. Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

Most manufacturers recommend 6 years. Sunlight degrades the rubber (which obviously won't be an issue in the boot well) but so does temperature change. Rubber will naturally change over time and personally I wouldn't worry up to 6 years, then after that periodically visually inspect the side walls for cracks or hardening tread blocks.
If I was in any doubt then for peace of mind and for the sake of £150 I would change it. _________________
Gone - 2010 Rimini XS